1. Field of the Invention
The present invention relates to a pulse spectrometer, and particularly to a pulse spectrometer which irradiates the measurement region of an in vivo tissue with light and conducts spectroscopic analysis for noninvasively determining the concentration of blood constituents.
2. Description of the Prior Art
In conventional noninvasive measurement of in vivo tissue blood constituents by near infrared spectrometry the measured value tends to fluctuate with the amplitude of the pulsation at the measurement region. The measurement therefore has to be conducted over a long period for averaging out the effects of the pulsation.
When in vivo tissue is irradiated with light, the transmission of the light through the tissue is accompanied by light absorption by the blood and light absorption by the tissue. In noninvasive measurement of blood component concentration, the measurement accuracy is degraded by the tissue light absorption.
Water has a particularly large effect on measurement accuracy because it makes up 50-60% of body tissue and has a major absorption peak at wavelengths in the vicinity of 980 nm, which is the wavelength region in which the absorption peaks of most of the blood constituents to be measured fall.
These circumstances make it necessary to eliminate the effect of the tissue so as to increase the relative amount of light absorbed by the blood constituents. For eliminating the effect of the tissue, it is advantageous to extract the amount of change in light absorbance of the blood caused by pulsation. Moreover, the fact that concentration measurement requires an absolute quantity of blood makes it essential to measure the amount of absorbance.
The pulse oximeter was developed on the basis of this knowledge to positively utilize blood pulsation for obtaining spectroscopic information from blood in an in vivo tissue. The pulse oximeter measures the oxygen saturation degree of blood hemoglobin by using the change in absorbance caused by pulsation as a signal source and measuring the amount of change at each of two wavelengths during each segment of a subdivided pulse period. Since this method can determine the oxygen saturation degree by a comparison between two frequencies, the calculation uses only the change in the amount of absorption by the blood.
As this signal processing method ignores the total amount of absorption, however, it cannot measure the hemoglobin concentration of the blood and other such concentrations that relate to the absolute value of the light absorbance. It is therefore not appropriate for analyzing blood component concentration.
Japanese Patent Laid-open Publication No. Hei 4(1982)-60650 teaches another device that utilizes pulsation by using a blood pressure and blood oxygen saturation degree measurement technique based on what is known as the volumetric vibration method. This device applies cuff pressure to the measurement region for reading the light absorbances of the tissue, vein and artery layers in various combinations and then calculates the oxygen saturation degrees of the arterial blood and veinal blood by determining the difference in light absorbance of the combinations at two wavelengths, and further measures the blood pressure from the cuff pressure at the change points of the pulse amplitude.
Regarding the application of cuff pressure, Japanese Patent Laid-open Publication No. Hei 5(1993)-503856 teaches a method of determining arterial blood oxygen saturation degree and arterial blood pressure in which the light absorbance of the arterial blood is measured under a cuff pressure approximately equal to the arterial pressure so as ascertain the change in light absorbance with pulsation at maximum amplitude.
The prior art devices for analyzing in vivo tissue blood constituents by near infrared spectrometry utilize only measured change in the arterial signal and difference in light absorbance. As a result, they are capable of determining only the blood oxygen saturation degree and do not employ any means or method for utilizing the light absorbance value or other blood component concentration data.